1. Field of the Invention
The invention relates to a wide-angle catoptric focal system, in particular for astronomical space observation over a broad spectral range.
2. Background Information
Catoptric light may be thought of as a light in which the rays are concentrated by reflectors into a beam visible at a distance. Thus, a catoptric system may be thought of as a system that relates to mirrors and reflected images. Typically, such systems have a combination of mirrors that may be used in astronomical or space observations.
Combinations of mirrors that constitute a wide-angle system are known, for example as described in the publications EP-A-0 601 871 and U.S. Pat. No. 5,379,157 which relates to four-mirror systems including a convex primary mirror making it possible to obtain a field of view of at least 13.5xc2x0 by 27xc2x0.
The present invention seeks to provide a four-mirror system making it possible to obtain a significantly wider field of view, of the order of 70xc2x0 by 2xc2x0, at least.
The invention also seeks to provide a system with low distortion.
The invention also seeks to provide a system that is compact.
This is achieved by the present invention by using a convex mirror as the secondary mirror.
In a typical embodiment of a combination of the invention, the combination comprises in succession:
a convex primary mirror (M1);
a convex secondary mirror (M2);
a tertiary mirror (M3) close to the focal plane; and
a concave quaternary mirror (M4).
The optical system of the invention can also be defined as the combination of a convex entrance mirror (M1) and a three-mirror anastigmatic system (M2, M3, M4) in which the first mirror (M2) is also convex.
The pupil, which may limit any flux collected by the optical system, is placed on the second mirror (M3) of the anastigmatic system.
It can also be defined backwards as the combination of the two mirrors (M4, M3) acting rather like a Schmidt telescope, the quasi-plane mirror (M3) serving as a pupil and being placed in the vicinity of the focus of M4 to obtain a quasi-telecentric configuration, together with two successive convex mirrors (M2, M1) which provide the desired large field.
The four mirrors are preferably aspherical so as to correct distortion and other aberrations.
The assembly comprising the objective lens, the mirrors, and the structure is preferably made of the same material, such as aluminum, Invar, or a ceramic having a low coefficient of expansion.
A conventional focal system may include a focal plane and a focal length. A focal plane may be thought of as a plane (through the focal point) at right angles to the principal axis of a mirror. Moreover, the focal plane may be that surface on which a bent image is formed. In this respect, a focal length may be viewed as the distance from the surface of a mirror to its focal point, where the focal point may be viewed as a point at which rays of light or other radiation converge or from which they appear to diverge. Thus, according to another feature of the invention, the distance between the primary and secondary mirrors and the distance between the secondary and tertiary mirrors lies in the range twice to seven times the focal length of the optical system as a whole, such as the focal length of the combination of mirrors M1, M2, M3, and M4.